This invention relates generally to etching processes in semiconductor wafer processing and more specifically to solder bump processing in flip chip technology.
The formation of integrated circuit (IC) chips involves many processes. From design to production, each aspect requires considerable attention to detail and precision to create and maintain a high yield of functioning chips. One such aspect is the method employed for interconnection and packaging of a patterned wafer die.
A traditional method of input-output (I/O) interconnection is wirebonding. This involves the bonding of I/O pads around the periphery of the wafer die with wires on a leadframe, followed by encapsulation into a chip package or packaging in a cavity-type package. An example of a typical wirebond is illustrated in FIG. 1a. As shown, a die 10 is bonded to a leadframe or substrate surface 12 by a wire bond 14. While this is a satisfactory implementation, the wafer area required for the bond pads restricts full utilization of wafer dies with increasing feature density on a single die.
Another form of interconnection is known as tape automated bonding or TAB bonding. In this method, the lead frame is bonded to a die by means of a tape placed over the die. One side of the tape goes over the die, while the other side of the tape goes on the leadframe or interconnects to a substrate board. Contacts on the die are plated up, i.e., grown above the circuit, to form "bump" contacts on the IC so that the passivation layers on the die are not cracked by the application of the tape. While this has been used successfully for many years in semiconductor manufacturing, there are risks of cracking the die during the tape application and forming sites of destruction, so that some fabrication facilities have begun to phase it out for cost-effectiveness reasons. Cost is also a factor since custom tapes are needed for each specific chip design.
Alternatively, flip chip technology provides an increased density of possible interconnections on a single die without risking the integrity of the die. Flip chip devices implement connections directly on the wafer so that the wafer can be "flipped" over, as the name implies, and bonded directly to a substrate, such as bonding to trace pattern wiring pads on a printed circuit board. The bonding is done via solder bumps, i.e., soldered areas formed on top of the die. FIG. 1b illustrates a chip 16 having solder bumps 18 connecting it to a substrate 20. It is the integrity of these solder bumps that determines the integrity of the interconnection and chip functionality for a given use.
In prior art practices, these solder bumps are formed by depositing a layer of solder into openings over bond pads on a patterned wafer and then etching the areas outside these soldered bond pads to leave the solder bumps 18. Solder deposition includes using evaporation methods, which are expensive and time-consuming, or using plating techniques. The etched areas are typically multiple metal cathode layers each of which requires a separate etching and cleaning step for removal. For example, aluminum, nickel, and copper are typical metals used for the metal layers. Accordingly, a standard etchant for the copper and nickel layers is a nitric acid solution, while a strongly phosphoric acid (&gt;80% phosphoric acid) solution is commonly used for the aluminum layer. These etchants have been found to attack the applied solder, making them less than ideal for solder bump formation. Further, each time the wafer is etched and cleaned to remove each of the metal layers, there is an increased risk of contamination and degradation of the solder bumps, so that every added step increases production time and cost. Therefore, in a continuing effort to improve upon solder bump formation, the present invention uses a dilute phosphoric acid solution to remove the metal layers in a more efficient and effective manner without degrading the solder layer. In this way, a simpler, one-step etching process results, which offers less waste production and less chemical consumption.